Wavelength division multiplexed (WDM) optical communication systems are known, in which multiple optical signals, each having a different wavelength, are combined onto a single fiber. In such systems, the wavelength of each optical signal is typically controlled to be at or close to a particular value. Otherwise, if the wavelengths were permitted to drift, two or more optical signals may have the same wavelength and interfere with each other, resulting in unacceptable data transmission errors. Additionally, use of preset assignments for signal-bearing wavelengths enables effective use of preset optical filters in the transmission systems.
In order to increase the capacity of WDM optical signals, the wavelength spacing associated with the optical signals may be reduced so that more optical signals can be combined onto an optical fiber. With smaller spacings, however, the wavelength of each optical signal may more readily drift into that of another optical signal. Accordingly, the wavelengths in such higher capacity WDM systems may need to be more tightly controlled than those in lower capacity systems having few optical signal wavelengths.
Conventional wavelength locking schemes may use an etalon to lock an optical signal to a particular wavelength. The etalon, however, has a periodic transmission characteristic including a plurality of transmission peaks. Accordingly, it is possible for an optical signal wavelength to “hop” from one wavelength associated with one of the transmission peaks of the etalon to another wavelength associated with an adjacent transmission peak. In that case, the conventional wavelength locker would lock to the optical signal to the wrong wavelength, potentially resulting in two optical signals having the same wavelength. The optical signals or channels would therefore conflict resulting in disrupted data transmission.
Moreover, WDM optical communication systems may include a chain of optical amplifiers that provide gain to the optical signals. In such systems, if the optical signals are amplified unevenly, the over-amplified signals will receive more gain than the under-amplified signals as the optical signals propagate through the amplifier chain. As a result, the under-amplified signals may lose power and may not be adequately detected. Accordingly, the power levels of each optical signal are often adjusted to be substantially uniform in order that one signal is not amplified more than the other signals. The optical power associated with each optical signal is therefore monitored to insure that each optical signal has a desired power level.
Accordingly, there is a need for improved wavelength and power monitoring in a WDM optical communication system.